Storm Sewer Model Calibration For Reduced Capacity

I am currently developing and calibrating a storm sewer model to account for reduced flow capacity based on CCTV Serer Video Inspection Reports.
I also have rainfall and flow monitoring data to calibrate to.
Results of the CCTV Sewer Video Inspection Reports indicate that pipe flow capacities may be reduced from 10% to 35% for select pipes within the system.
In order to calibrate to the observed flow monitoring data, should I be modeling pipes with a reduced diameter to reflect the decreased flow capacity, or should I be modeling pipes with an increased Manning's roughness coefficient to reflect the decreased flow capacity?
Any references to published material, or personal experience would be greatly appreciated.

 

[n3rei]

Well I think you can answer your own question by knowin what is causing the decrease in flow reduced diameter and/or increased mannings. As a help Generally over time mannings n does increase due to scumming roughing etc. in the pipes, but if the before mentioned also has decreased the diameter than include that in your comps. Also see if the full flow capacity will be affected. If only the channel low flow is scummed of rough then make an adjust ment to the n for that portion only.